Advanced silicon bipolar, CMOS or BiCMOS circuits are used today for high-speed applications in the 1–5 GHz frequency range, replacing circuits previously only possible to realize using III–V based technologies. Their major application area is for modern telecommunication systems. The circuits are used mostly for analog functions, e.g. for switching currents and voltages, and for high-frequency radio functions, e.g. for mixing, amplifying, and detecting functions.
To obtain transistors well suited for e.g. telecommunication applications, not only a low transit time (high fT) is needed, but also a high maximum oscillation frequency (fmax), and good linearity are required. Today's silicon bipolar junction transistors (BJT) technology can offer fT up to 50 GHz, but is reaching its physical limitations because of the trade-off between the thickness and resistivity of the base layer.
By adding some (typically 10–20%) germanium into the base of a conventional BJT, the high-frequency characteristics can be improved substantially. The new device is an SiGe (silicon germanium) HBT (heterojunction bipolar transistor) structure.
The base layer structure is usually grown with MBE (Molecular Beam Epitaxy) or CVD (Chemical Vapor Deposition), but it is also possible to implant germanium into the silicon, but with less control of the doping profile. During recent years, SiGe-based transistors have shown record high-frequency performance with regards to fT and fmax (maximum oscillation frequency), see “Enhanced SiGe Heterojunction Bipolar Transistors with 160 GHz-fmax” by A. Schüppen et al., IEEE IEDM Tech Dig., p. 743, 1995. For “high-frequency applications, e.g. wireless communication, the SiGe HBT can be used to boost performance of existing double-polysilicon RF-ICs and BiCMOS technologies. An extensive review of SiGe epitaxial base technology is given in “SiGe HBT Technology: A New Contender for Si-Based RF and Microwave Circuit Applications” by J. D. Cressler, IEEE TED-46, p. 572, May 1998.
SiGe can be added into existing IC-process flows in different ways. Some typical examples of extending a BiCMOS process with SiGe-base transistors can be found in “BiCMOS6G: A high performance 0.35 μm SiGe BiCMOS technology for wireless applications” by A. Monroy et al., IEEE BCTM 1999, p. 121 and in “A 0.24 μm SiGe BiCMOS Mixed-Signal RF Production Technology Featuring a 47 GHz FtHBT and 0.18 μm Leff CMOS” by S. A. St. Onge et al., IEEE BCTM99, p. 117, 1999.
A simpler, yet feasible method to fabricate high-performance SiGe HBT transistor, is by using epitaxial deposition of the device layers, and then form the device structure by mesa transistor etching, similar to fabrication of compound semiconductor devices (e.g. GaA1As HBTs). The mesa structures have been widely used to quickly verify concepts and explore device characteristics because of its simplicity and ease of fabrication, see “Si/SiGe HBTs for Applications in Lower Power ICs” by D. Behammer et al., Solid-State Electronics, Vol. 39, No. 4, p. 471, 1996.
IC-type of circuits generally require more complicated structures than a few transistors, and the mesa concept, discussed in the previous section, is generally not suitable for this. With refined fabrication schemes, such as described in U.S. Pat. No. 5,587,327 to U. König et al. and in U.S. Pat. No. 5,821,149 to A. Schüppen et al., some of the drawbacks can be circumvented. However, a few critical process steps still remain, such as the differential epitaxy (simultaneous epitaxial growth on silicon substrate openings, and deposition of non-epitaxial material on field areas and other structures), and the critical removal of the part of the emitter layer on the extrinsic base areas, which make the concept less feasible for high-volume semiconductor production.
A simpler method to realize and integrate a mesa-type of SiGe HBT transistor into a semiconductor process flow suitable for high-volume production is therefore needed.